mk: Fix cross-compiling to armv7-unknown-linux-gnu
mk: Fix cross-compiling to armv7-unknown-linux-gnu

The cross prefix was not likely the actual compiler that needed to be used, but
rather the standard `arm-linux-gnueabihf-gcc` compiler can just be used with
`-march=armv7`.

graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust